1. Field of the Invention
The present invention generally relates to a multi-target manipulator for a pulsed laser deposition apparatus. More particularly, it relates to a multi-target manipulator which grows a thin film having both a high quality and a repeatability, automatizes a deposition apparatus, and economically performs deposition, when a thin film is grown by using a pulsed laser deposition apparatus.
2. Description of the Conventional Art
In thin film deposition methods for growing a thin film on a single crystal substrate and an amorphous substrate, a pulsed laser deposition (or a laser ablation) which forms a thin film having a constant thickness on a semiconductor substrate by using the atomic vapor generated by applying a pulsed laser to a target material is one of thin film fabrication methods by a physical method generally used.
The pulsed laser deposition apparatus used in the pulsed laser deposition method includes:
a laser generation apparatus for generating a laser light having a wavelength range of 100-400 nm; and
a target driving apparatus for driving a target material;
an electric heater for attaching and fixing a substrate and a heat treatment. Herein, the target driving apparatus and the electric heater are installed in a vacuum chamber.
A thin film growth using the pulsed laser deposition apparatus is achieved in the reactive gas atmosphere of hundreds of mTorrs, uses oxygen as the reactive gas in oxidized substances, and uses argon as the reactive gas in metallic materials and high-polymer materials.
In order to deposit the target material on a substrate using a pulsed laser deposition method, high-energy laser beam having an ultraviolet wavelength is used as an energy source. Such a laser beam is focused at the target material within the vacuum chamber through both a focusing lens and a quartz window, after being generated from the laser generation apparatus.
At this time, the condensed area on the target material occupies tens of mm.sup.2. In this way, the laser beam of high-energy condensed on such a narrow area melts the target material, thus generates a laser plume of atomic vapor. The vaporized atoms flies to a substrate being faced forward the target at the speed of tens of kilometers per an hour. Likewise, the atoms arrived at the substrate form an atomic layer deposited on the substrate by a self-chemical reaction and a reaction between the atoms and the substrate constituent atoms. The atomic layer has the same composition as the target material maintaining at a minimum binding energy. In this case, if the substrate is exposed to the laser plume for a predetermined time, then thin film having a constant thickness can be grown on the substrate.
The position of a focused laser beam in a laser deposition apparatus is varied according to the position of a quartz window through which the laser beam is passed. The focused area on a target material is varied depending on a focal length and a position of the focusing lens.
Accordingly, selecting properly the quartz window and the focusing lens is very important for condensing the laser beam energy to a fixed point inside a vacuum chamber. At the focal point, a deposited material is vaporized by the highly condensed laser energy.
In order to grow a high-quality thin film by using the target material with which a deposition is performed, the target material should be positioned near the focal point of a fixed laser beam within the vacuum chamber. In forming the thin film on the substrate, a reproducibility of the thin film is a very important parameter in addition to the high-quality characteristic thereof. To achieve the reproducibility, a mechanical accuracy which repeatably positions the target on a predetermined laser focal point should be assured. As a result, the target manipulator which can precisely drive a given target is required.
Generally, four targets or more targets can be mounted on this target manipulator. The reason why several target materials are mounted on the target manipulator is for an advantage wherein a multilayer thin film is manufactured as many as the number of allowable targets (four or more) without break the vacuum when the system is under vacuum.
Also, the design is very useful when a buffer layer is firstly formed on the substrate before growing the main thin film or when a special thin film such as an artificial superlattice structure is formed on a substrate.
As described above, in case a plurality of targets are used, the target manipulator should be designed to perform two kinds of motions as follows:
Firstly, a motion mechanism should move a specific target on a focal point of the laser beam. In this motion mechanism, a plurality of targets (i.e., a multi-target) on the target disk are arranged on a single circumference with an equal distance therebetween, and the focal point of the laser beam are positioned on one point of the circumference. Accordingly, since the specific target can be moved on the focal point of the laser beam when the disk rotates, the target being deposited can be selected.
Secondly, the target should be rotated. The deposition can be started when the laser beam is positioned on one predetermined target. However, if the deposition is performed on only one fixed point of the target, a composition transformation of the material occurs because of a heat concentration by a high laser energy concentrated on the fixed point, thereby this composition transformation directly exerts a bad influence upon the composition of the thin film grown on the substrate.
In addition, when a polishing is performed on the target material in order to attain the same composition as the material in the next deposition run, the target material (commonly, a target's diameter is over 25 mm) of area beside the focal point is discarded. Therefore, in order to eliminate the aforementioned problems, the target should be rotated.